Personally I don't have much OpenGL programming experience, so I don't really know to what Nvidia FSAA mode this corresponds. Perhaps I'm talking bull**** but you might be using an FSAA mode that isn't supported by your card or else Nvidia is using different algorithms to get similar FSAA results. Have you already experimented with the __GL_FSAA_MODE environment variable to see if those settings have similar impact on cpu-useage?